SPI Exam Study Guide - Sonography Principles & Instrumentation

Comprehensive vocabulary flashcards covering the five domains of the ARDMS SPI exam content outline, including physical principles, artifacts, instrumentation, Doppler concepts, and safety/QA.

Musculoskeletal injury

The most common occupational hazard for sonographers, often caused by repetitive strain, improper wrist positioning, or poor ergonomics.

Reflection

The phenomenon where sound bounces off a boundary between two media with different acoustic impedance ($Z$).

Snell's Law

The law governing refraction, which is the bending of the sound beam when it crosses an interface at an oblique angle where propagation speeds differ.

Scattering

Sound redirected in many directions by small or rough reflectors, such as red blood cells; it serves as the basis for Doppler signals.

Absorption

The conversion of sound energy into heat, which is the primary cause of attenuation in soft tissue.

Attenuation

The overall loss of intensity and amplitude as sound travels through tissue, calculated as the sum of absorption, scattering, and reflection.

Attenuation Coefficient (Soft Tissue)

A rule of thumb estimated at approximately $0.5\,dB/cm/MHz$ for one-way travel.

Acoustic Impedance ($Z$)

The product of tissue density ($\rho$) and propagation speed ($c$), represented by the formula $Z = \rho \times c$.

Reverberation

An artifact characterized by equally spaced, repeating echoes caused by sound bouncing between two strong parallel reflectors.

Shadowing

The loss of echo signals deep to a strongly attenuating or reflecting structure, such as a stone or bone.

Enhancement

Also known as posterior or through transmission, it is an increase in brightness deep to a weakly attenuating structure like a cyst.

Mirror Image Artifact

A duplicate display of a structure on the far side of a strong reflector, commonly seen with the diaphragm and liver.

Speed Error Artifact

An artifact where a structure is displayed at the incorrect depth because the actual propagation speed differs from the machine's assumed $1,540\,m/s$.

Range Ambiguity

An artifact occurring when a returning echo from a previous pulse is misinterpreted as coming from the current pulse's depth, often seen in high PRF Doppler.

Mechanical Index ($MI$)

An estimate of the likelihood of cavitation, calculated as $MI = \frac{p(r)}{\sqrt{f}}$, where $p(r)$ is the peak rarefactional pressure and $f$ is frequency.

ALARA Principle

Stands for 'As Low As Reasonably Achievable,' guiding the use of the lowest possible output and exposure time for diagnostic quality.

Voxel

A 3D volume element, the three-dimensional equivalent of a 2D pixel.

Piezoelectric Element (PZT)

A ceramic or single-crystal material that converts electrical energy into sound during transmission and vice versa during reception via the piezoelectric effect.

Matching Layer

A component that reduces impedance mismatch between the element and skin to improve sound transmission; its ideal thickness is $\frac{1}{4}$ wavelength.

Backing Material

Also called the damping layer, it reduces pulse duration and spatial pulse length to improve axial resolution but decreases sensitivity.

Axial Resolution

The minimum distance between two reflectors along the beam axis needed to see them as separate, calculated as $\frac{SPL}{2}$. Smaller values indicate better resolution.

Lateral Resolution

Resolution perpendicular to the beam axis, which is equal to the beam width and is best at the focal zone.

Temporal Resolution

The ability to accurately depict motion over time, represented by the frame rate.

Pulse Repetition Frequency ($PRF$)

The number of pulses sent out per second, determined solely by imaging depth.

Nyquist Limit

The maximum Doppler shift that can be detected without aliasing, equal to $\frac{PRF}{2}$.

Harmonic Imaging

A technique that processes the second harmonic frequency ($2 \times \text{fundamental frequency}$) to reduce near-field artifacts and improve resolution.

Duty Factor

The fraction of time the transducer is transmitting, calculated as $\text{Pulse Duration} \div PRP$.

Dynamic Range

The range of echo intensities the system can handle, where increasing it results in more shades of gray and a lower-contrast image.

Doppler Equation

The formula used to calculate Doppler shift: $\Delta f = \frac{2 \times f_0 \times v \times \cos(\theta)}{c}$.

Wall Filter

A high-pass filter designed to remove low-frequency, high-amplitude signals caused by vessel wall or tissue motion.

BART

A mnemonic for color Doppler convention: Blue Away, Red Toward the transducer.

Aliasing

A wrap-around artifact in pulsed Doppler that occurs when the Doppler shift exceeds the Nyquist limit ($PRF/2$).

Power Doppler

A Doppler mode that displays the amplitude/strength of the signal rather than velocity or direction; it is angle-independent and highly sensitive to low flow.

Bernoulli Principle

A hemodynamic principle stating that as blood velocity increases at a stenosis, the pressure simultaneously drops.

Resistive Index ($RI$)

A calculation used to quantify downstream vascular resistance: $RI = \frac{PSV - EDV}{PSV}$.

Sensitivity

The ability of a test to correctly identify those with a disease (true positive rate), calculated as $\frac{TP}{TP + FN}$.

Specificity

The ability of a test to correctly identify those without a disease (true negative rate), calculated as $\frac{TN}{TN + FP}$.